1. Field of the Invention
The invention relates to superconductive circuits and comprises a novel SQUID (Superconducting Quantum Interference Device) construction. The invention is particularly applicable to utilization in Josephson circuits.
2. Description of the Prior Art
Said Ser. No. 836,452 discloses a SQUID configuration for use in Josephson circuits. The present invention is also a SQUID device having similar utilization but providing enhanced properties with respect thereto. The background prior art with respect to both devices is as follows.
Superconductive Josephson logic and memory circuits are known in the art wherein the active switching element or gate is the conventional Josephson tunnel junction. The Josephson junction comprises two superposed layers of superconductive material with an insulator layer therebetween sufficiently thin to support the Josephson tunneling effect. As is known, Josephson tunnel junctions suffer from numerous disadvantages. While the superconductive metals such as lead, indium and tin or alloys thereof exhibit suitable superconductive properties, these materials cause difficulties when fabricating superconductive integrated circuits utilizing fine line interconnections. When utilizing standard lithographic processes to produce such circuits, the metal layers tend to move and form hillocks during heating at temperatures greater than 70.degree. C., which temperatures are normally required in performing the printed circuit fabrication operations. Printed circuit fabrication techniques that tend to partially ameliorate the difficulties tend to be more complicated than more conventional fabrication techniques that presently produce high yield integrated circuits of a more conventional variety such as semiconductor circuits. In place of the relatively low melting point superconductive metals mentioned, aluminum has been utilized as the superconductor to alleviate the problem but aluminum has a significantly lower superconductive transition temperature than, for example, lead and therefore requires more energy to refrigerate the devices.
The conventional Josephson tunnel junction requires an insulating layer sufficiently thin (5-30 Angstrom units) to support Josephson tunneling. Utilizing present day integrated circuit fabrication techniques, it is exceedingly difficult to produce such thin layers with uniform thickness. As is known, the zero voltage Josephson current through the tunnel junction depends strongly upon the thickness of the tunneling insulator barrier, the dependence being at least as strong as exponential. Thus, small variations in barrier thickness produces large variations in the zero-voltage Josephson current. This results in the undesirable effect of Josephson junctions having wide variations in properties which adversely affect the yield of integrated circuits utilizing such elements.
An additional disadvantage of the Josephson tunnel junction is that spurious unwanted resonance states can occur that tend to disrupt the desired operation of the junction. In order to alleviate this problem prior art techniques utilize critical control of dimensional parameters and shapes of the junctions necessitating undesirably complex fabrication techniques.
Additionally, methods of fabricating niobium-niobium pentoxide-niobium tunnel junctions have been devised although such junctions have not to date been utilized in memory or logic circuits. It is recognized that although such junctions utilize refractory superconductive materials, thus not suffering from the disadvantages discussed above with respect to the low melting point materials, an extremely difficult fabrication procedure is required. Furthermore, such junctions suffer from the above described disadvantages with respect to Josephson tunnel junctions.
With the exception of the device of said Ser. No. 836,452, it is believed that heretofore only Josephson tunnel junctions have been utilized or considered as the active switching element for superconductive logic and memory circuits. Although the Josephson tunnel junction has had adequate sensitivity in control of the critical current by means of overlying control lines it is desirable to further enhance the control sensitivity of the switching element so as to further improve the parameters of the circuits in which the switches are utilized. The SQUID of said Ser. No. 836,452 achieves this objective and the SQUID of the present invention retains this advantage as well as providing additional enhanced characteristics.
Additionally, the "weak-link" Josephson device or microbridge is known in the prior art primarily in the context of utilization in SQUID magnetometers. Such weak-links or microbridges are generally difficult to fabricate requiring expensive time consuming detailed contouring by electron beam etching. Such devices are generally planar SQUIDs with the weak-links parallel to the device ground plane, which configuration would render the magnetic field of an overlying control line ineffective in regulating the critical currents. Although the SQUID arrangement of plural microbridges provides great sensitivity of critical current with respect to the magnetic fields in which the magnetometer may be immersed, a single weak-link is substantially insensitive to control by magnetic fields since it is the magnetic flux linking the area of the weak-link that provides the control and the area of the weak-link necessarily is exceedingly small. Thus such planar SQUIDs and single weak-links have not heretofore been considered useful as the active switching element in Josephson circuits which in practical arrangements require magnetic field control via overlying control lines.
SQUID devices are also known in the art that utilize Josephson tunnel junctions as the active switching elements. Such devices form the macro memory and logic elements which themselves require the utilization of the Josephson tunnel junction as the active switching elements thereof.
The SQUID of the present invention as well as that of said Ser. No. 836,452 may be advantageously utilized, inter alia, as the replacement for the Josephson tunnel junction as the active switching element for utilization in logic and memory circuits.
Although the SQUID of said Ser. No. 836,452 is eminently suited for utilization in Josephson circuits, the device suffers from several disadvantages. The critical current of the device of said Ser. No. 836,452 at 4.2.degree. K. is generally undesirably large for device dimensions which can be readily fabricated. Typically currents of 50-100 milliamperes are experienced. This undesirably large critical current results in the dissipation of significant additional power when the device is switched out of the zero voltage state. Since it is necessary that cryogenic circuits exhibit low power consumption to have practical utility, this additional power dissipation results in a disadvantage of the device of said Ser. No. 836,452. The large critical current of the device of said Ser. No. 836,452 could be reduced by operating the device nearer the superconductive transition temperature. However, this is not of practical utility, the boiling point of liquid helium (4.2.degree. K.) being considered the most convenient operating point for most superconductive devices.
Since the weak-links of the device of said Ser. No. 836,452 are formed with superconductive metal, they may not be sufficiently "weak" for the link dimensions that may practically be fabricated. This results in a reduced sensitivity to applied magnetic control fields and a reduction in the percentage modulation of the critical current by the applied magnetic field (as generally discussed in said Ser. No. 836,452). This limitation results in exceedingly stringent requirements on the precise control of current levels throughout the circuitry utilizing such devices. Alternatively, to obtain the required "weakness" of the links in said Ser. No. 836,452, narrow holes of significant depth are required in forming the links, a requirement that is exceedingly difficult to achieve utilizing practical fabrication techniques considering the submicron dimensions required.
It is a desideratum of the present invention to provide a SQUID device that eliminates the above described disadvantages of the SQUID of said Ser. No. 836,452 while retaining the advantages thereof.